The keto analog of L-leucine (a dietarily essential amino acid) is .alpha.-keto-isocaproic acid, which is usually referred to as ketoisocaproate (KIC), or sometimes also as ketoleucine. In the accepted description of leucine metabolism, leucine is first transaminated to its ketoacid, o-ketoisocaproate (KIC). KIC then enters the mitochondria and is decarboxylated to isovalarylCoA by the branched chain ketoacid dehydrogenase. [See Krebs, et al., Adv. Enz. Reg. 15:375-394 (1976); and Paxton, et al., J. Biol. Chem. 257:14433-14439 (1982).] An alternate minor pathway has been described in the rat and human liver [Sabourin, et al., Fed. Proc. 38:283 Abs. 312 (1979)]. This alternate oxidative pathway occurs in the cytosol and involves oxidation of KIC to .beta.-hydroxy-.beta.-methyl butyrate (HMB) by the enzyme KIC-oxygenase [Sabourin, et al., Arch. Biochem. Biophys. 206:132-144 (1981)]. According to Sabourin et al. (1981, p. 143): "The importance of this pathway is presently unknown".
The administration of keto analogs of amino acids has been proposed for treatment of certain disease conditions in humans, such as uremia. [See, for example, Walser, et al., J. Clin. Inv., 52:678-690 (1973).] For nutritional purposes, it is known that KIC is an inefficient substitute for leucine. Rat studies have shown that the feeding of KIC as a replacement for leucine requires the feeding of from two to three times as much KIC as the nutritionally required amount of leucine: Chawla, et al., J. Nutr., 105:798-803 (1975); and Boebel, et al., J. Nutr., 112: 1929-1939 (1982); and Chow, et al., J. Nutr., 104: 1208-1214 (1974).
It has been proposed to feed small amounts of KIC in conjunction with animal diets containing sufficient leucine for the purpose of improving the growth metabolism of the animals. By using milligram amounts of KIC, some increases in the rates of weight gain and/or feed efficiencies have been obtained. [See, for example, U.S. Pat. Nos. 4,760,090 and 4,883,817.] With mature sheep being fed for wool production, by feeding KIC the amount of wool produced may also be increased (U.S. Pat. No. 4,760,090). When lactating domestic animals, such as dairy cattle, are fed small amounts of KIC, the quantity of milk produced may be increased (U.S. Pat. No. 4,758,593). Egg production by laying chickens can also sometimes be increased (U.S. Pat. No. 4,760,531). Other effects of feeding KIC have been observed, including cholesterol reduction in meat, milk and eggs (U.S. Pat. Nos. 4,760,090 and 4,760,531), and some apparently beneficial effects on the immune system (U.S. Pat. No. 4,835,185).
Why KIC addition to protein and leucine sufficient animal diets can have different effects than the metabolic conversion of leucine to KIC is not known. The beneficial effects of KIC on domestic animals are not consistently obtainable. Variability of benefits have been particularly observed in the KIC feeding of ruminants. KIC supplementation has not become an established animal feeding practice for any purpose. With reference to the use of KIC in feeds for cattle and sheep, a major problem is that the KIC is subject to partial rumen destruction, which is a variable factor. The use of rumenprotective agents and procedures for KIC has not been shown to be satisfactory for commercial purposes.
Nutritionally, as described above, leucine is converted to KIC, which in the mitochondria is decarboxylated to isovalarylCoA. In certain disease conditions, such as isovalaric acidemia, an alternate oxidative pathway for KIC has been observed in the liver, which may produce the substance .beta.-hydroxy-.beta.-methylbutyrate (HMB). To date, however, there is little or no evidence that HMB is normally produced in the metabolism of KIC. In some extreme cases, such as genetic absence of the dehydrogenase enzyme, there is evidence HMB accumulates in the urine: Tanaka, et al., Proc. Natl. Acad. Sci., 56:236-242 (1966); and Tanaka, et al., Biochim. Biophys. Acta, 152:638-641 (1968). In acidosis conditions HMB levels may be increased in urine: Landass, Clin. Chim. Acta, 64:143-154 (1975).
Animals can be stimulated to grow in a general way which increases all organs and tissues. In that case, overall weight gain is usually increased, and feed efficiency can also be increased, although usually not as much as the rate of gain. Muscle or lean tissue can be stimulated to grow at the expense of fat or other organs. In that case, weight gain rate may not change, but usually feed efficiency is improved. Thus, growth of lean tissue can be markedly stimulated without a major change in average daily gain, or other measure based on body weight. The meat industry is now moving toward paying producers on the basis of lean tissue weight rather than on total animal weight. Thus, it is becoming important to base performance on lean tissue gain and lean tissue feed efficiency. However, previous research with KIC had not shown that it specifically stimulates lean tissue gain (e.g., muscle growth). In some cases KIC has been shown to decrease fat deposition, such as in sheep, but KIC generally has not been found to stimulate muscle growth.